This invention relates to dimmable electronic ballasts for particular use with fluorescent lamps.
Fluorescent lamps are controlled by an electronic ballast that provides the necessary ignition voltage for the discharge that provides the illumination. Since a minimum voltage must be provided for the ignition to start, providing a means for dimming the voltage is difficult, as simply varying the supply voltage (which would be effective with say a conventional filament light bulb) cannot be used without jeopardising the ignition voltage.
Notwithstanding this difficulty providing a dimmable ballast is desirable for a number of reasons, including the desire of energy efficiency, and the desire to create a pleasant working environment. A range of techniques have been developed to enable a dimmable fluorescent lamp to be created. For example dimming may be provided by controlling the switching frequency so that the impedance of the limiting inductor in the ballast can be varied so as to control the current into the lamp. Another way of providing dimming control is to control the duty-cycle of the switching devices (switching at constant frequency) so that the average applied voltage and current in the lamp can be varied.
Moreover, of particular importance in the design of electronic ballasts are the requirements for high power factor, low total harmonic distortion, low electromagnetic interference (EMI), low lamp current crest factor, and low flickering. In order to comply with the requirements of certain international standard (for example IEC1000-3-2 Class C appliances), commercially available products usually consist of two cascaded stages for the input power factor correction (PFC) and output high-frequency inversion. A regulated dc voltage, for example 400V, inter-links the two stages. Some specialized integrated circuits have been developed for this particular application to simplify the circuit schematics. For the input stage, boost- and flyback-type pre-regulators are the most popular choices. For the output stage, either a voltage-fed or current-fed inverter is usually chosen. A resonant tank circuit is used because of its distinct advantages of near-sinusoidal lamp current and high voltage generation during the ignition period. The dimming operation is based on adjusting the switching frequency of the inverter so that the reactance of the series inductor can be varied and thus the lamp power can be controlled.
In order to simplify the overall circuitry and reduce the manufacturing cost, many passive PFC circuits and single-stage electronic ballasts with dimming features have recently been reported. Some of them minimize the circuit structure by integrating the switch in the PFC into one of the switches in the inverter. However, their structural elegance is offset by various problems, such as high lamp current crest factor, asymmetrical lamp voltage and current waveforms, high dc link voltage at low luminous level, high component stresses, and narrow dimming range. In order to achieve a desired dimming range and soft-switching of the switches in the inverter, the switching frequency of the inverter has to be varied in a single-stage system. This can result in lamp current and voltage waveform distortion. The dc link voltage has to be set to a much higher value, which can be two to three times higher than the rated value of the supply voltage for reducing the above problem. This aspect is particularly problematic for countries having high-voltage mains supply, such as 220V in Hong Kong and the UK, resulting in the requirement for a very high voltage dc link capacitor for stabilizing the inverter input.
Furthermore, existing dimmable ballasts are generally four-wire systemsxe2x80x94two (conventional live and neutral) for the ac supply mains input and the other two connecting to a variable dc voltage input (for example 0-10V) or variable resistor for dimming control. Recently, a ballast that can provide a dimming features using a TRIAC-based dimmer at the supply mains has been proposed in xe2x80x9cPhase-controlled dimmable electronic ballast for fluorescent lampsxe2x80x9d of W. Ki, J. Shi, E. Yau, P. Mok, and J. Sin, in Proc. IEEE Power Electron Spec. conf. 1999 pp1121-1124. However, the dimmer has to be modified so that the actual firing angle is varied within few degrees of angle only. It has been proposed in xe2x80x9cTRIAC dimmable integrated compact fluorescent lampxe2x80x9d of J. Janczak in J. of Illuminating Eng. Soc., pp144-151 1998 to use an ordinary dimmer to adjust the luminous level of compact fluorescent lamps. The dc link voltage is kept at a relatively constant value throughout the dimming range. Again, changing the switching frequency of the inverter varies the output luminous level, and the dimming range is relatively narrow.
Despite the various proposals discussed, there exists a need for a simple yet effective electronic ballast overcoming the drawbacks discussed.
In one aspect the invention provides an electronic ballast comprising:
limiting means for receiving, in use, an alternating current and providing user adjustment of the angular range of switch-on in each alternating current cycle;
an ac-dc rectifier receiving said user-limited alternating current and outputting a dc;
a dc-dc power converter adapted to provide voltage step up or step down of said rectified dc and to provide power factor correction; and
an inverter operated at constant frequency to convert the output of the power converter a high frequency alternating voltage.
The limiting means is preferably a TRIAC, allowing user-adjustment of the firing angle over a wide range. The ac-dc rectifier may be a simple full-wave diode bridge rectifier. A filter is provided in order to remove unwanted high frequency noise from the dc-dc power conversion.
The converter may take a variety of forms, for example, a flyback converter, operated in discontinuous conduction mode, in order to ensure that the filtered input current profile is phase-coincident with the TRIACxe2x80x94controlled voltage waveform. A converter control circuit provides pulse width modulation of the converter duty cycle whilst maintaining constant switching frequency.
In a further aspect of the invention, the dc-dc power converter is adapted to provide power factor correction and to effect voltage step up or down of said rectified dc signal in dependence on the magnitude of the rectified voltage according to a predefined algorithm or equation.
The said predefined algorithm or equation is arranged to effect a mapping between the ballast output power and the firing angle of the TRIAC. This mapping may be a linear relationship but could take other forms.
For example, the mapping may be realised by a linear relationship effected by stepping up or down the dc voltage according to the following function:       v          EQ      ,      out        *    =            1      2        ⁢          (              1        +                              1                          K              1                                ⁢          ln          ⁢                      xe2x80x83                    ⁢                                                    K                2                            +                              v                                  EQ                  ,                  in                                                                    1              +                              K                2                            -                              v                                  EQ                  ,                  in                                                                        )      
where
xcexdxe2x88x92EQ,out is an approximated value of the required voltage after mapping.
xcexdEQ,in is the average input voltage of the rectified TRIAC controlled voltage
and k, k2 are constants affecting the accuracy of the approximation.
A variety of means of realising the mapping can be utilised, including an analogue circuit, or an active microprocessor providing real-time or near real-time computation, or a memory mapping technique wherein there are stored values of output voltages corresponding to input voltages.